Although a MOPA configuration is in principle more complex than a laser which directly produces the required output power, the MOPA concept can have certain advantages:

With a MOPA instead of a laser, it can be easier to reach the required performance e.g. in terms of linewidth, wavelength tuning range, beam quality or pulse duration if the required power is very high.
This is because various performance aspects are decoupled from the generation of high powers.
This gives extra flexibility, e.g. when a gain-switchedlaser diode is used as a seed laser.
Note also that it can be advantageous to avoid the presence of additional optical components such as wavelength tuning elements in a high-power laser resonator; with a MOPA architecture, one can place these in the oscillator, where they do not have to withstand high optical intensities, do not spoil the power efficiency, etc.

The same aspects apply to other kinds of modulation, e.g. intensity or phase modulation: it may be advantageous to modulate the low-power seed laser, or to use an optical modulator between seed laser and power amplifier, rather than to modulate a high-power device directly.
Slower power modulation may be done by adjusting the amplifier's pump power, without significantly affecting e.g. the obtained pulse duration or wavelength.

The combination of an existing laser with an existing amplifier (or an amplifier chain) may be simpler than developing a new laser with higher output power.

The optical intensities are lower in an amplifier, compared with the intracavity intensities in a laser.

However, the MOPA approach can also have disadvantages:

The complexity of the setup is higher.

The wall-plug efficiency is often lower.
However, it may also be higher, e.g. if that approach allows to remove lossy optical elements from the high-power stage.

The resulting laser noise tends to be higher, since an amplified source can not reach the shot noise level (→ amplifier noise).
Effects of drifts of the seed power may be suppressed, however, if the amplifier is operated in a strongly saturated regime.

A MOPA can be sensitive to back-reflections, which are amplified again before entering the master laser.
This feedback sensitivity can often be cured only by placing a Faraday isolator behind the amplifier.
Particularly for high-power pulsed devices, this can introduce serious limitations.

MOPA architectures are also used for pulsed laser sources.
In that case, the amplifier may be used as a reservoir of energy.
If a pulse from the seed laser extracts a significant fraction of the stored energy, the effect of gain saturation is relevant: the amplifier gain drops during the pulse.
This can lead to a deformation of the temporal pulse shape.
In some cases, the pulse shape from the seed source is tailored so as to obtain the desired pulse shape after amplification.

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